About the Brain
Ever known a psychopath? You probably have but just didn't know it. Most psychopaths, about 500,000 in the U.S., end up in jail. However, experts estimate that there are another 250,000 that live in society as best they can.  Researchers call these "successful psychopaths."
One of the most disturbing aspects of my adult life was knowing a psychopath quite well, although I didn't know the correct label at the time. I was never worried for my own safety but noticed that this human being was completely self-centered and had virtually no sympathy for other human beings, including his own family.
And these are some of the hallmarks of a true psychopath: they have very low empathy and emotions. They simply do not "feel" the suffering of other human beings as a normal human being would. In fact, they have low levels of all emotions. They also are responsive to typical risk and reward scenarios and are known for fearlessness and impulsiveness.
This combination of lack of fear, lack of empathy and low emotion is what turns so many of them into killing machines. They can kill, rape and even torture, without remorse and without regret. The person that I knew was not a killer but had multiple wives in multiple states simultaneously and had little to no remorse for the incredible damage that he left behind.
Now I mention all of this, not to let you know that I am vying for a screenwriting job on Criminal Minds, but because an examination of psychopathology shows the incredible importance of guarding our gray and white matter. As I will show below, psychopathology is an extreme, but straightforward lesson in how we should treat the most important organ in our body: the brain.
What neurobiologists have found, through neuroimaging and other recent techniques, is that the brain of the typical psychopath has been partially destroyed. There is an inner donut, if you will, of cerebral subsytems called the paralimbic system, that governs "decision making, high-level reasoning and impulse control."  The paralimbic system includes the anterior cingulate, orbitofrontal cortex, amygdala, poserior cingulate, insula, etc. In psychopaths, these all-critical regions have actually been "shrunk" or reduced in volume.  Or they may have never fully developed due to a traumatic childhood or genetic factors.
Furthermore, there is another part of the brain that is probably also tied into psychopathology: the prefrontal cortex. The prefrontal cortex is the part of the frontal cortex that is involved in things like suppressing urges, deciding between conflicting thoughts and so on. Damage to the prefrontal cortex from aging and other factors is what makes being around older men so scary at times: they just blurt out whatever they are thinking.
The prefrontal cortex is supposed to act as the filter, if you will, for what we say and do. However, in some older men this filter has been effectively removed due to literal brain damage and so you get to hear their every thought.
It may be not suprise that prefrontal atrophy or damage like this has been tied to criminal psychopaths. Those psychopaths that end up incarcerated have been found to have 21% less volume in this key area of their brain  and, of course, it shows in their actions.
Now here is what should make us all pause and reflect societally and otherwise. Consider some of the factors that can damage the prefrontal cortex according to the research:
1. Physical Trauma. A blow to the head, especially during childhood. 
2. Cortisol. It is well-know that elevated cortisol levels, such as those in patients with PTSD and/or depression, can literally unhook neurons and shrink the hippocampus. However, one groundbreaking 2010 study found evidence that elevated cortisol was also associated with atrophy of prefrontal tissues as well. 
3. Alzheimer's. If you've been on a typical Western Diet for very long, then the typical plaques and tangles of Alzheimer's have been accumulating in your brain due to elevated inflammation and other factors. As the decades roll by, the damage begins to manifest itself in the typical signs of dementia that we associated with Alzheimer's. But it's important to note that you can have substantial plaques and tangles before you really start to reach "critical mass" and exhibit symptoms. One 2010 study showed a correlation between the damage of Alzheimer's and "prefrontal volume."  This means that Alzheimers leads to a shrinkage of the prefrontal cortex.
So, yes, every morning you should wake up and ask yourself, "What am I doing to my poor brain?" Do you want to be one of those grumpy, old guys that no one wants to be around and that embarasses himself much of the time?
Furthermore, there is a sobering study out there that shows that healthy people can potentially be shifted toward psychopathology through frontal lobe damage.  Of course, I am not saying that damaging your frontal/prefrontal regions is going to automatically turn anyone into a serial killer. But let's be honest: damage to the brain has consequences. It has consequences to our behaviors, our relationships and our careers. Again, this is yet another reason to carefully guard and protect your gray matter.
Another question should be asked as well: "Why is it that so many psychopaths come from unspeakably awful family backgrounds?" Well, the jury is still out, but one possible explanation is that extreme childhood stress, and its ensuing elevated cortisol levels, leads to a permanently damaged and defective brain in the prefrontal and paralimbic regions. There are undoubtedly genetic, hormonal - there are many fewer females psychopaths and females do not move to psychopathology as easily - and neurological factors as well of course.
Are you in a toxic, highly stressful or abusive relationship? If so, you need to find a way to reduce your physiological stress reaction. Are you depressed? Are you suffering from sleep deprivation or apnea? Again, do what you have to do: you've only got so much gray and white matter and there is no uglier example than that of a psychopath to remind us of that fact.
1) Scientific American Mind, "Inside the Mind of a Psychopath", Sep/Oct 2010, p. 28.
2) ) Scientific American Mind, "Inside the Mind of a Psychopath", Sep/Oct 2010, p. 27.
3) Biological Psychiatry, Nov 1 2001, 50(9):677-684, "Limbic abnormalities in affective processing by criminal psychopaths as revealed by functional magnetic resonance imaging"
4) Biological Psychiatry, May 15 2005, 57(10):1103-1108, "Volume Reduction in Prefrontal Gray Matter in Unsuccessful Criminal Psychopaths"
6) Neuroimage, 2010 Nov 15, 53(3):1093-102, Epub 2010 Feb 13, "Salivary cortisol and prefrontal cortical thickness in middle-aged men: A twin study"
7) Neurobiology of Aging, Nov 1999, 20(6):591-596, "Sex differences in prefrontal volume with aging and Alzheimerï¿½s disease"
8) Cognitive and Behavioral Neurology, July 2000, 13(3)"The Relation Between Tendency for Psychopathology and Reduced Frontal Brain Volume in Healthy People"